Liquid and gas separation processes are well known in the art. Most common separation processes involve a phase change, which increases the cost of the processes. Membrane separations, however, can achieve desired levels of separation without a change in the substances' phase. In essence, membrane separation selectively forces one or more substances through pores of a filter, leaving one or more larger substances behind. This is repeated until a satisfactory level of separation is achieved.
As is also known in the art, a satisfactory level of separation depends on the nature of the substances involved. For example, the purification of water has very strict governmental requirements to insure that public safety hazards are avoided. Industrial wastewaters must meet standards for a host of chemicals and compounds, including heavy metals and organics, before being allowed to enter public sanitary sewer systems.
Because of the increased popularity of membrane separation, there are a plurality of devices currently being used. For example, spiral wound membrane devices, sheet membrane devices, and straight tube membrane devices are all currently being used to achieve membrane separation. A spiral membrane device is constructed using a flat sheet of polymeric membrane, which, together with a mesh-type spacer is rolled around a perforated hollow tube. The liquid, therefore, must pass through several layers of membrane material before entering the perforated tube and exiting the separation device.
While the spiral membrane devices work well for some applications, they do have some limitations, such as susceptibility to fouling and, to a lesser degree, to extremes of temperature and pressure. Fouling is basically the clogging of the membrane over the duration of its use. Although the mesh-type spacer that is utilized reduces fouling, it can not totally eliminate this problem. Further, one method of restoring the integrity of the membrane is to apply a solution with either a high or low pH. This practice, due to its inherent corrosivity, reduces the life of the membrane.
Simliarly, sheet membrane devices work well in some applications, but have limitations, such as low membrane area and time consuming maintenance. Due to the size of these devices, it is difficult to provide the same membrane area as other membrane configurations. For example, a series of large flat sheets consume a larger space than a spiral wound membrane. Further, the cleaning and changing of the system involve disassembly of a large number of components.
Straight tube membrane devices work well in some applications, but have limitations, such as low membrane area and, to a lesser degree, fouling. In this configuration several long straight tubes, often made from ceramic materials, are housed in a large pipe. The materials used can usually withstand wider temperature and pH ranges but their shape and orientation does not allow for a large membrane area. There have been recent innovations which have attempted to increase the membrane area of the ceramic type of this class of devices; using one piece of straight ceramic tube and constructing numerous small "tubes" which run along its length. However, the performance of this configuration is limited by the pronounced fouling resulting from the small size of the "tubes".
Therefore, a need exists for a membrane separation method and apparatus that can withstand extremes in temperature and in pH, reduce the effect of fouling, while minimizing spatial requirements and maximizing specific membrane area.